Liquid crystal displays have been widely used in various mobile devices, owing to its high image quality, compact size, and light weight. In those devices, such as in cell phones and personal digit assistants (PDAs), a pure transmissive (T) typed LCD that exhibits a high contrast ratio and good color saturation is usually employed. However, the pure T type LCD has a poor outdoor readability because its image is washed out by strong ambient lights, such as sunlight, which makes it imperfect for devices that require frequency outdoor applications. Besides, its dependence on a backlight causes a relatively high power consumption, which further reduces the working time for each battery change. On the other hand, a reflective (R) type LCD does not require a backlight unit and uses the ambient light for displaying images instead. As a result the display's power consumption is low and has a good outdoor readability, making it uniquely useful for certain environments. However, the unavoidable surface reflection inherent in R type devices result in an inferior contrast and color saturation as compared to the T mode LCDs.
To overcome abovementioned problems of both the transmissive mode and reflective mode displays in mobile device applications, the transflective typed LCD combines both T and R type LCDs into one display to obtain environment-versatility, good image quality, and low power consumption. A transflective type LCD can operate in a transmissive mode and/or a reflective mode. In one example, each pixel of the transflective LCD is divided into a T sub-pixel and a R sub-pixel, and the cell gap of the T (dT) and R (dR) regions are different (dT˜2 dg) for maximum light efficiency as described in U.S. Pat. No. 6,341,002 issued to Shimizu et al. on Jan. 22, 2002.
However, besides the light efficiency, another critical issue in abovementioned kind of display in its dependence on a broadband circular polarizer. As shown in FIG. 1a, a typical broadband circular polarizer 10 in most prior art transflective LCDs consists of one linear polarizer along with one mono-chromatic half-wave plate and one mono-chromatic quarter-wave plate under a special alignment as described by S. Pancharatnam (“Achromatic combinations of birefringent plates: part I. An achromatic circular polarizer,” in Proc. Indian Academy of Science, vol. 41, sec. A, (1955), pp. 130-136. The mono-chromatic half-wave plate has its optics axis set at 15 degrees with respect to the transmission axis of the top polarizer, and the mono-chromatic quarter-wave plate has its optics axis set at 75 degrees with respect to the transmission axis of the polarizer. Because of the existence of the broadband quarter-wave plate above the LC cell for R mode, the T mode requires another circular polarizer to be set below the LC cell to achieve a common dark state as in the R mode. A problem with this configuration is that the viewing angle of the transflective LCDs is quite narrow.
FIG. 1b is a graph 20 that shows the simulated light leakage of two stacked circular polarizers, in which the light leakages at different viewing angles, both azimuthal and polar directions are calculated correspondingly. The calculated results are normalized to its maximum possible output value between two parallel aligned linear polarizers in the normal direction. As shown in FIG. 1b, the light leakage of two stacked broadband circular polarizers is severe at off-axis, e.g., the approximately 10% light leakage occurs within a cone at 40 degrees, which means the 10:1 contrast ratio of two stacked circular polarizers is limited to approximately 40 degrees. However, the corresponding angular light leakage for two crossed linear polarizers is much less as shown in the graph 30 in FIG. 1c. The 10% light leakage is well suppressed to over 80 degrees, and 1% light leakage is over 50 degrees.
What is needed to solve these problems is a transflective structure with a high light efficiency, good gray scale overlap between T and R modes, but no dependence on the abovementioned circular polarizer. In addition, a single cell gap structure for the purpose of simple fabrication and good yield is also of great interest to those skilled in the art.